I have a hard time picturing how a "return to nomadic tribes" would even work. How would you turn a densely populated country like, say, the Netherlands, into nomadic tribes? The planet could never sustain 8 billion people living as nomads. Not to mention, it wouldn't stop war or conquest in any way. See the Mongols, Scythians etc.
Areas with lots of city states have historically been hotbeds of war and chaos. Ancient Greece or the HRE constantly had tiny states squabbling. It would also make large infrastructure projects much harder. Building a highspeed railway line through Europe is hard enough right now, imagine the same thing but instead of 5 countries, it goes through 50. It would also be a mess to get any unified environmental policies through.
So no, I don't think either option is realistic or desirable.
We don't need to ban every single bit of plastic. It's fine to keep it where it's absolutely necessary. Some of the examples you provided definitely aren't necessary though, like individually wrapped cutlery (wtf) or beverages (can use glass).